Musannaf Ibn Abi Shayba 9:1550

Asbat ibn Muhammad narrated to us, from Ash'ath, from al-Hasan, who said: "The woman experiencing postpartum bleeding shall not remain [in that state] for more than forty days." 'Ata' said: "She shall remain for the duration of her habit that she was accustomed to, and she shall not remain for more than forty nights."